The winter-rainfall region of southern Africa, covered largely by the fynbos and succulent karoo biomes, harbours the world’s greatest concentration of geophyte species. Species diversity is greatest in the south-west, where more than 500 species co-occur in one quarter-degree square; in the south-east the values are generally around 100, and in the arid north-west, always less than 50 (more often less than 10). In at least three species-rich genera (Moraea, Eriospermum and Oxalis), the size of storage organs (bulbs, corms, tubers) varies inversely, with the largest average values occurring in the species-poorer areas — both in the north-western, and in the south-eastern parts of the region. This negative correlation between average storage organ size and species diversity is, however, only observed at relatively large spatial scales, which suggests that there is no direct relationship between storage organ size and species diversity. More likely, both these measures are driven by winter rainfall amount and reliability, both of which peak in the south-western Cape. We suggest that reliable winter rainfall makes large storage organs unnecessary and depresses extinction rates, thus leading to the accumulation of species. Keywords: Bulbs, Cape Floristic Region, geophytes, Namaqualand, southern Africa, species diversity.